[cfe-dev] Even abs() comes with a performance penalty

[Stefan Kanthak via cfe-dev]

"Craig Topper" <craig.topper at gmail.com> wrote:

> Sorry. I made a mistake. Cmov has been 1 cycle since Broadwell.

Doesn't matter, no need to worry: all instructions used below run in 1 cycle on
recent CPUs ... just like Jcc. The question/point is but whether the CPU can/does
speculate ahead.

Stefan

On Sun, Sep 6, 2020 at 12:39 PM Craig Topper <craig.topper at gmail.com> wrote:

> cmov has been 1 cycle since Sandy Bridge. Moves execute in the register
> renamer since Ivy Bridge. So mov+neg+cmov should be faster than cdq+add+xor
> on modern CPUs. Furthermore, cdq really ties the hands of the register
> allocator so probably doesn't make sense in a larger function with abs
> mixed with other code.
>
> ~Craig
>
>
> On Sun, Sep 6, 2020 at 12:30 PM Stefan Kanthak via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> --- bugs-bunny.c ---
>> // Copyleft © 2014-2020, Stefan Kanthak <stefan.kanthak at nexgo.de>
>>
>> #ifdef __amd64__
>> __int128_t __absti2(__int128_t argument) {
>>     return argument < 0 ? -argument : argument;
>> }
>> #else
>> long long __absdi2(long long argument) {
>> #ifdef BUNNY
>>     return __builtin_llabs(argument);
>> #else
>>     return argument < 0 ? -argument : argument;
>> #endif // BUNNY
>> }
>>
>> long __abssi2(long argument) {
>> #ifdef BUNNY
>>     return __builtin_labs(argument);
>> #else
>>     return argument < 0 ? -argument : argument;
>> #endif // BUNNY
>> }
>> #endif // __amd64__
>> --- EOF ---
>>
>> Run clang -c -o- -O3 -S -target amd64-pc-linux bugs-bunny.c
>>
>> Left: inperformant original code # right: proper code,
>>                                  #        faster and 3 bytes shorter
>>
>> __absti2:      # @__absti2
>> # %bb.0:                         # .intel_syntax noprefix
>>       xorl     %edx, %edx        #        mov    rax, rsi
>>       movq     %rdi, %rax        #        cqo
>>       negq     %rax              #        mov    rax, rdx
>>       sbbq     %rsi, %rdx        #        add    rdi, rdx
>>       testq    %rsi, %rsi        #        adc    rsi, rdx
>>       cmovnsq  %rdi, %rax        #        xor    rax, rdi
>>       cmovnsq  %rsi, %rdx        #        xor    rdx, rsi
>>       retq                       #        ret
>>
>> CMOVcc introduces a data dependency here, WITHOUT necessity!
>>
>>
>> Run clang -c -o- -O3 -S -target i386-pc-linux bugs-bunny.c
>>
>> Left: inperformant original code # right: proper code, runs even on real
>>                                  #        i386, not just PentiumPro+
>>
>> ___abssi2:    # @__abssi2
>> # %bb.0:                         # .intel_syntax noprefix
>>       movl    4(%esp), %ecx      #        mov    eax, [esp+4]
>>       movl    %ecx, %eax         #        cdq
>>       negl    %eax               #        add    eax, edx
>>       cmovll  %ecx, %eax         #        xor    eax, edx
>>       retl                       #        ret
>>
>>
>> Writing shorter code for __absdi2() for i386 is left as an
>> exercise to the reader.
>>
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